Shea Charles' 85th-minute winner broke the fragile hearts of Gunners' fans and sent Southampton on an unlikely trip to Wembley, fifty years after they won the famous trophy for the only time in their history.
Arsenal dominated possession, as expected. They had travelled to the south coast without some key players, but would have felt that they still had enough quality to deal with the Saints, who have an even bigger match at Wrexham on Tuesday.
Southampton, though, proved a tough nut to crack. The spirit of '76, embodied in a cauldron of noise and passion, inspired a team that was never going to just roll over.
Peretz has become Southampton's MVP
In goal, Daniel Peretz was a goliath. Both full-backs made life difficult for Gabriel Martinelli and Max Dowman (who was fantastic by the way). Taylor Harwood-Bellis and Nathan Wood stood up to the test of Gabriel Jesus well and celebrated like they had won the cup at the end of the match.
Caspar Jander proved to anybody who might be in any doubt that he belongs at a higher level. He will probably return to Germany in the summer unless the Saints can get themselves promoted. On last night's form, promotion is not beyond this group of players and coaches.
Cameron Bragg came of age, chasing Martin Odegaard, Kai Havertz and, in the latter stages, Martin Zubimendi around for 90 minutes. Bragg, a student of the game, will have learned plenty from that experience.
Finn Azaz is another whose stage should be the Premier League. Southampton's playmaker showed once again that he can unlock the best of defences, creating a chance and playing three balls into the final third. Probably unnoticed was that Azaz made the most recoveries of any outfield player last night. His defensive positioning is uncanny. He just seems to be a magnet for loose balls!
Scienza at his eye-catching best after injury
Léo Scienza was Southampton's most eye-catching player. He always is. A pathetic dive for a penalty showed his worst side early in the first half. A few minutes later, he ran through on goal but couldn't quite get his shot away. And, in the second half, he tormented Ben White before crashing a right-footed shot against the Arsenal crossbar.
Scienza was withdrawn after 70 minutes, enabling him to start against Wrexham on Tuesday night. Many clubs would have taken notice of the Brazilian's impact.
On the right wing, Tom Fellows won the battle with Myles Lewis-Skelly. Fellows probably should have got his first goal for the Saints, but the assist for Shea Charles' winner will be more than adequate consolation.
Described unflatteringly as the Loch Ness Drogba, Ross Stewart got the last word with a well-taken goal in the first half. He judged the flight of the ball, on what was a windy night on the south coast, better than White and, after taking the ball down on his chest, thumped the ball low and hard past Kepa.
The substitutes made a huge difference, coming on just after Arsenal had equalised. Charles added energy to the midfield as Bragg began to fade a little. Edozie added defensive effort to the left side as Arsenal threatened. Cyle Larin did what he does best and provided an escape route for a defensive unit that was getting its sternest test of the season.
Joshua Quarshie's late introduction was more about killing the game than anything else.
Player ratings:
Daniel Peretz - 9
James Bree - 8
Ryan Manning - 8
Taylor Harwood-Bellis - 7
Nathan Wood - 8
Caspar Jander - 8
Cameron Bragg - 7
Finn Azaz - 7
Léo Scienza - 8
Tom Fellows - 8
Ross Stewart - 8
Cyle Larin - 8
Shea Charles - 9
Sam Edozie - 7
Joshua Quarshie - N/A
Tonda Eckert (Head Coach) - 10